1. Field of the Invention
This invention relates to superconducting oxide articles having improved characteristics for alternating current operation and to a method for manufacturing them. In particular, the invention relates to composite articles comprising multiple substantially electrically decoupled domains, each containing one or more fine filaments of a superconducting oxide material, and to methods and intermediates for manufacturing such composites.
2. Background of the Invention
Since the discovery of the first oxide superconductors less than a decade ago, there has been great interest in developing high temperature superconducting conductors for use in high current applications such as power transmission cables, motors, magnets and energy storage devices. These applications will require wires and tapes with high engineering critical current densities, robust mechanical properties, and long lengths manufacturable at reasonable cost. Superconducting oxide materials alone do not possess the necessary mechanical properties, nor can they be produced efficiently in continuous long lengths. Superconducting oxides have complex, brittle, ceramic-like structures which cannot by themselves be drawn into wires or similar forms using conventional metal-processing methods. Moreover, they are subject to a magnetic effect known as flux jumping which causes sudden localized temperature variations that can force them out of their superconducting state if it is not compensated. Consequently, the more useful forms of high temperature superconducting conductors usually are composite structures in which the superconducting oxides are supported by a matrix material which adds mechanical robustness to the composite and provides good thermal dissipation in the event of flux jumping. The matrix material chosen must be readily formable, have high thermal conductivity, and be sufficiently non-reactive with respect to the superconducting oxides under the conditions of manufacturing and use that the properties of the latter are not degraded in its presence. For composites made by the popular powder-in-tube or PIT process, described, for example, in U.S. Pat. Nos. 4,826,808, and 5,189,009 to Yurek et al. and W. Gao and J. Vander Sande, Superconducting Science and Technology, Vol 5, pp. 318-326, 1992; C. H. Rosner, M. S. Walker, P. Haldar, and L. R. Motowido, xe2x80x9cStatus of HTS superconductors: Progress in improving transport critical current densities in HTS Bi-2223 tapes and coilsxe2x80x9d (presented at conference Critical Currents in High Tc Superconductorsxe2x80x2, Vienna, Austria, April, 1992) and K. Sandhage, G. N. Riley Jr.,. and W. L. Carter, xe2x80x9cCritical Issues in the OPIT Processing of High Jc BSCCO Superconductorsxe2x80x9d, Journal of Metals, 43,21,19, all of which are herein incorporated by reference, the matrix material must also provide sufficient oxygen access during manufacturing to allow the formation of a superconducting oxide from its precursor material. Very few matrix materials meet these requirements. Under normal manufacturing conditions, superconducting oxides have adverse reactions with nearly all metals except the noble metals. Thus, silver and other noble metals or noble metal alloys are typically used as matrix materials, and pure silver is the matrix material generally preferred for most high performance applications although composite matrices, including, for example oxide diffusion barriers or silver layers between superconducting oxides and non-noble metals have been suggested in the prior art.
Many of the superconductor applications that have the greatest potential for energy conservation involve operating the superconductor in the presence of an AC magnetic field, or require that the superconductor carry an AC current. In the presence of time-varying magnetic fields or currents, there are a variety of mechanisms that give rise to energy dissipation, hereafter called AC losses, even in superconductors. Thus, the superconductor geometry must be selected to reduce AC losses, in order to preserve the intrinsic advantage of superconductors, the absence of DC electrical resistance. The physics governing AC losses in low temperature superconducting composite materials have been described and analyzed, c.f. Wilson, Superconducting Magnets, Ch 8(1983,1990), W. J. Carr, Jr., AC loss and macroscopic theory of superconductors, Gordon and Breach Science Publishers, New York, 1983, and would be expected to operate in superconducting oxide composites with similar geometries. In general, the primary sources of AC loss are hysteretic loss within the superconducting filament(s),and eddy current loss in the matrix enhanced by coupling between superconducting filament(s). To minimize hysteretic losses, the superconductor is preferably subseparated into many small filaments that are discrete and dimensionally uniform along the length of the conductor. Eddy current losses may be minimized by increasing the electrical resistivity of the matrix or by twisting the filaments, with tighter twist pitches providing lower losses. However, the inherent chemical and mechanical limitations of superconducting oxide composites limit the degree to which these approaches may be relied on for reducing AC losses in high temperature superconducting composites. Conventional methods for increasing the resistivity of the matrix have also been limited. Silver, the matrix material of choice for these composites for the reasons discussed above, has a very low electrical resistivity. Efforts have been made to increase the resistivity of the matrix, for example, by distributing small amounts of oxide-forming metals in finely separated form in a silver matrix, and by using higher resistivity alloys to form all or part of the matrix adjacent to the filaments. However, the presence of even very small amounts of chemically reactive materials near the filament/matrix boundary during the manufacturing process can significantly degrade the properties of the superconducting oxide composite. This is a particularly delicate issue for composites consisting of many fine filaments as the higher surface to volume ratio greatly increases the risk of contamination. In the xe2x80x9cPITxe2x80x9d manufacturing process, layers of high resistivity material can also block oxygen access to the filaments during manufacturing, inhibiting the formation of the superconducting oxide from its precursors. In addition increasing the electrical resistivity of the matrix adjacent to the filaments, whether by surrounding the filaments with a resistive layer or by providing a uniformly doped matrix, generally decreases its thermal conductivity, increasing the risk of flux jumping during use.
Thus, an object of the invention is to provide multifilamentary superconducting composite articles in any desired aspect ratio with improved AC loss characteristics and high critical current densities, and a method for manufacturing them.
Another object of the invention is to provide a method of reducing coupling losses in multifilamentary superconducting oxide composite articles without significantly increasing the risk of contamination of the superconducting filaments by the supporting matrix.
Another object of this invention is to provide a method of manufacturing superconducting composite articles suitable for AC applications which provides adequate oxygen access for formation of a desired superconducting oxide with optimal current carrying capacity.
Another object of the invention is to provide highly aspected multifilamentary BSCCO 2212 and 2223 composite conductors having high current densities, superior AC loss characteristics and robust mechanical properties, and a method for producing them.
In one aspect, the invention provides a multifilamentary superconducting composite article comprising multiple substantially electrically decoupled domains, each including one or more fine, preferably twisted filaments comprising a desired superconducting oxide material. Tapes, wires and other elongated multifilamentary articles are preferred forms of the article. In a preferred embodiment, the article comprises a matrix, at least one discrete filament decoupling layer comprising an insulating material, which is disposed within the matrix to separate the matrix into a plurality of substantially electrically decoupled domains; a plurality of filaments, each comprising a desired superconducting oxide, which are disposed within or around and preferably essentially encapsulated by the matrix and chemically isolated thereby from the decoupling layers, each of the substantially electrically decoupled domains containing at least one filament. The invention provides reductions in coupling losses roughly proportional to the square of the article""s cross-sectional ratio. It facilitates the production of multifilamentary articles that exhibit good DC performance characteristics and markedly superior AC performance, particularly in highly aspected forms.
In another aspect, the invention provides an intermediate for a multifilamentary superconducting composite article comprising multiple domains, each including one or more fine, preferably twisted filaments of a superconducting oxide material. In a preferred embodiment, the intermediate comprises a matrix, at least one discrete filament decoupling layer comprising an insulating material or its predecessor, which is disposed within the matrix to separate the matrix into a plurality of substantially separate domains; a plurality of fine, preferably twisted filaments, each comprising a desired superconducting oxide or its precursors, which are disposed within or around and preferably essentially encapsulated by the matrix and chemically isolated thereby from the decoupling layers, each of the separate domains containing at least one filament.
By xe2x80x9cfilament decoupling layersxe2x80x9d are meant discrete layers comprising insulating materials or their predecessors, in geometric forms of sufficient dimension to significantly increase the resistance between domains in the finished article. In the preferred embodiment, each domain is at least partially bounded by the surfaces of one or more filament decoupling layers but the arrangement and materials of the layers are selected so they do not substantially inhibit oxygen access to the filaments in the domain during processing. Typically the filament decoupling layers extend parallel to the filaments along the length of the article, and are very thin in proportion to their width and length. In cross section, they may resemble, for example, fins, donuts, stars, centipedes and combinations of these. In the fully processed article, the thickness of the insulating material is less than the filament thickness, and preferably less than about 5 microns.
By xe2x80x9csubstantially electrically decoupled domainxe2x80x9d, as that term is used herein, is meant that the direct high conductivity path between adjacent domains is at least 50% and preferably 85% occluded by the filament decoupling layers, but not more than 100%, preferably not more than 99% and most preferably not more than 95% occluded.
By xe2x80x9cinsulating materialxe2x80x9d, as that term is used herein, is meant a material with an electrical resistivity high in comparison to that of the matrix material used in the composite under the intended conditions of use. Typically, the insulating material selected will have resistivity at least 10 times higher than that of the selected matrix material. Materials with resistivities greater than about 20 xcexc-ohm cm may be used, and materials with resistivities greater than about 100 xcexc-ohm cm are most preferred. Elemental oxides, sulfides, nitrides, semiconductors, intermetallics and other non-metallic insulating materials are suitable. By xe2x80x9cpredecessorxe2x80x9d, as that term is used herein, is meant any material that can be converted to an insulating material by heat treatment under suitable conditions. Metals with high oxidation rates, particularly the transition metals, the alkaline earths, thallium, zirconium, niobium, molybdenum, aluminum and their alloys are preferred predecessor materials, and zirconium, niobium, molybdenum, hafnium, tantalum, tungsten, titanium, vanadium, manganese, cobalt, iridium, vanadium nickel,iron and chromium are particularly preferred. Zirconium, niobium, nickel,iron and molybdenum are most preferred.
By xe2x80x9cmatrixxe2x80x9d as that term is used herein, is meant a material or homogeneous mixture of materials which supports or binds a substance, specifically including the filaments, disposed within or around the matrix. By xe2x80x9cnoble metalxe2x80x9d, as that term is used herein, is meant a metal which is substantially non-reactive with respect to oxide superconductors and precursors and to oxygen under the expected conditions (temperature, pressure, atmosphere) of manufacture and use. xe2x80x9cAlloyxe2x80x9d is used herein to mean an intimate mixture of substantially metallic phases or a solid solution of two or more elements. Silver and other noble metals are the preferred matrix materials, but alloys substantially comprising noble metals, including ODS silver, may be used.
In a preferred embodiment, a conductive jacketing layer surrounds the article. Noble metals and alloys comprising noble metals, including ODS silver, are the most preferred jacketing layer materials, but other conductive materials, including composites of several different metals, may be used. Jacketing layers made from a material with a resistivity at least equal to that of the matrix material may be used, and jacketing layers with a resistivity on the order of about 0.5-10 xcexcohm are especially preferred.
By xe2x80x9cdesired oxide superconductorxe2x80x9d, as that term is used herein, is meant the oxide superconductor intended for eventual use in the finished article. Typically, the desired oxide superconductor is selected for its superior electrical properties, such as high critical temperature or critical current density. Members of the bismuth and rare earth families of oxide superconductors are preferred. By xe2x80x9cprecursorxe2x80x9d, as that term is used herein, is meant any material that can be converted to a desired oxide superconductor upon application of a suitable heat treatment. By xe2x80x9cfine filamentsxe2x80x9d are meant filaments with a cross-sectional dimension less than 750 and preferably less than 150 microns.
In yet another aspect, the invention provides a method of manufacturing a multifilamentary superconducting composite article having improved AC loss properties by first, forming a composite intermediate comprising multiple domains, each including one or more fine, preferably twisted filaments of a superconducting oxide material or its precursor, and then thermomechanically processing the intermediate at conditions sufficient to produce at least one of the effects of texturing, crack healing and, if a precursor to the desired superconducting oxide remains, phase transformation in the filament material under conditions which support the electrical separation of the domains. In the preferred embodiment, the forming step includes the step of providing filament decoupling layers comprising insulating materials or their predecessors to provide the desired domain separation, and in the most preferred embodiment, the insulating material is formed in situ from its predecessor during the thermomechanical processing step.
In one preferred embodiment, the forming step includes the steps of forming composite comprising a matrix, which substantially comprises a noble metal, a plurality of discrete filament decoupling layers disposed within the matrix to separate the matrix into a plurality of substantially separate domains, each layer comprising an insulating material or its predecessor, and a plurality of filaments, each comprising a desired superconducting oxide or its precursors, which are disposed within or around and preferably essentially encapsulated by the matrix and chemically isolated thereby from the decoupling layers, each of the domains containing at least one filament; and next, deforming the intermediate to produce at least one of the effects of twisting the filaments and texturing the material comprised therein. By xe2x80x9ctexturingxe2x80x9d, as that term is used herein, is meant inducing crystallographic alignment and intergrain bonding of the grains of a desired superconducting oxide or its precursors. In a preferred embodiment, the forming step includes the steps of forming filament decoupling layers from an oxide-forming predecessor to an oxide insulating material and the thermomechanical processing step comprises the steps of, first, heat treating the composite at conditions sufficient to passivate the predecessor and form an insulating material from at least part of the predecessor material but not to induce substantial phase transformation in the filament material; and, thereafter thermomechanically processing the composite at conditions sufficient to produce at least one of the effects of texturing, crack healing and, if a precursor to the desired superconducting oxide remains, phase transformation in the filament material. In a preferred embodiment, the process also includes the step of providing a conductive jacketing layer surrounding the article.